Effect of interfacial species mixing on phonon transport in semiconductor superlattices

Molecular dynamics simulations are used to examine the effect of interfacial species mixing on the thermal conductivity of Stillinger-Weber Si/Si_0.7Ge_0.3 and Si/Ge superlattices at a temperature of 500 K. The thermal conductivity of Si/Si_0.7Ge_0.3 superlattices is predicted to not depend on the interfacial species mixing and to increase with increasing period length. This period length dependence is indicative of incoherent phonon transport and related to decreasing interface density. The thermal conductivity of Si/Ge superlattices is predicted to depend strongly on the interface quality. For Si/Ge superlattices with perfect interfaces, the predicted thermal conductivity decreases with increasing period length before reaching a constant value, a trend indicative of coherent phonon transport. When interfacial species mixing is added to the model, however, the thermal conductivity is predicted to increase with increasing period length, indicating incoherent phonon transport. These results suggest that the assumption of coherent phonon transport made in lattice dynamics–based models may not be justified.